I'm able to access the web interfaces and to administer the mailing-lists.
I have created a mailing-list named 'test', but when i try to write to it, test@mydomain.com, it answers that the user test@mydomain.com doesn't exist.

I have read each and every post containing the word "mailman" in the forum before posting, but now i need some advice.
Being a novice about zimbra, it might be an obvious advice

I think I have reached the point where /opt/zimbra/postfix/conf/main.cf doesn't keep its configuration.